1. Field of the Invention
Present invention relates to a wireless communication system field based on cognitive radio, especially to a method for initializing protection device in the IEEE802.22.1 wireless microphone beacon system.
2. Description of the Related Art
With development of a technique of cognitive radio based on dynamic spectrum allocation and an application of equipments with unauthorized frequency bands, quick and reliable radio control channel is increasingly required. This control channel can be established with radio beacons. It is typically required that these beacons be robust to all kinds of radio channels. In addition, to avoid causing interference to authorized radio equipments or other unauthorized radio equipments, the beacon should be short with very low transmission power. And a beacon defined in IEEE802.22.1 is a typical one that meets demands mentioned above.
In general, two kinds of beacon transmission device, namely a primary beacon transmission device (PPD) and a secondary beacon transmission device (SPD) respectively, are defined in protocol IEEE802.22.1. PPD plays a leading role in controlling the transmission of the beacon. Therefore, in most cases, PPD transmits the beacon to SPD and a consumer premise equipment (CPE) in a wireless regional access network (WRAN). In some specified time slots, it is necessary for the SPD to intermittently transmit beacon data to PPD to allow the PPD to combine all beacon data so as to obtain information such as integral channel occupation ratio and a geography location and so on, and then broadcast it to the CPE.
A structure of a super frame defined in protocol IEEE802.22.1D1.0 is illustrated in FIG. 1. A super frame 101 contains 31 time slots. Each slot contains 32 DQPSK symbols. And a super frame is composed of two parallel logic channels. The first one is a synchronization channel 102. And the second is a beacon channel 103. The synchronization channel 102 includes a series of synchronization bursts. This super frame also contains a specified time slot, comprising an enough turn-round time for PPD and SPD to perform coding and decoding, a receiving period RTS (Request To Send) 104 and an ANP (Acknowledgement/No Acknowledgement Period) 105 for the transmission of ACK/NACK.
During the RTS period 104, the PPD monitors the beacon transmission request information from the SPD. And during the ANP 105, the PPD broadcasts determination information on whether the beacon information could be transmitted by SPDs or not and which SPD could perform this task.
It should be noted that when a PD is powered on, it is necessary to detect whether any PPD exists in the current channel or not. And this detect processing lasts 2+0.1 m (m∈[0,100]) seconds. Then an initialization process for a device is implemented in following two cases:
In the case that no PPD exists in current channel, the PD which is just powered on acts as a PPD and the device initialization process lasts 30 seconds. During the initialization process, the structure of the Beacon frame is illustrated in FIG. 2. The super frame is composed of two parallel logic channels. The first logic channel is a synchronization channel 202, and the second is a beacon channel 203. Differing from that shown in FIG. 1, the synchronization channel 202 occupies the entire 31 time slots, and the beacon channel 203 consists of 30 time slots of PPDU data and a slot of zero padding information.
In the structure of the Beacon physical layer PPDU frame shown in FIG. 3, a first byte 301 of a frame is PHR information, containing seven reserved bits 303 and one initialization bit 302. If PPD is in the initialization stage, which lasts 30 seconds, the initialization bit is set as “1” by a PPD high layer.
In the case that some PPD already exists in current channel, the PD which is just powered on acts as an SPD and receives the beacon frame from PPD. By checking whether the initialization bit is “1” or not, the SPD determines whether the PPD is in initialization stage or not.
In present IEEE802.22.1D1.0, only one bit is defined as the initialization bit. And how to correctly receive this initialization bit information is one of the future tasks of the present standard. In present task force, some relevant solutions have been proposed on this problem. For example: Samsung company puts forward that the initialization bit is repeated in a frame to determine whether the PPD is in initialization stage or not by the rule that most information bits are correctly received in the receiver, while Huawei Corporate suggests that several continuous correct initialization bits be received to determine whether the PPD is in initialization stage or not in the premise that number of initialization 1 bit is kept unchanged in the PHR of the beacon frame.
In solution 1, i.e., to repeat the initialization bit in a frame, the implementation is very simple and easy, but no obvious improvement for the receiver in regard to the initialization 1 bit in the case that the channel varies slowly or a signal-to-noise ratio is very low;
In solution 2, i.e., to receive the initialization bits in several continuous frames, accuracy of the judgment can be improved. However, more receive delay is introduced in the system so that complexity is increased in the receiver.
By far, no final solution is approved by IEEE802.22TG1 task force.